docs: def relation_ids(reltype=None):
      """A list of relation_ids"""
      reltype = reltype or relation_type()
      relid_cmd_line = ['relation-ids', '--format=json']
      if reltype is not None:
          relid_cmd_line.append(reltype)
          return json.loads(
              subprocess.check_output(relid_cmd_line).decode('UTF-8')) or []
      return []